US4973242A - Metal mold for injection molding tubular or columnar products and molded product obtained by using same - Google Patents

Metal mold for injection molding tubular or columnar products and molded product obtained by using same Download PDF

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Publication number
US4973242A
US4973242A US07/333,250 US33325089A US4973242A US 4973242 A US4973242 A US 4973242A US 33325089 A US33325089 A US 33325089A US 4973242 A US4973242 A US 4973242A
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United States
Prior art keywords
tubular
cavity
molded product
metal mold
mold
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Expired - Fee Related
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US07/333,250
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English (en)
Inventor
Mitsuhiro Tsukada
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Polyplastics Co Ltd
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Polyplastics Co Ltd
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Assigned to POLYPLASTICS CO., LTD., 3-13, AZUCHI-MACHI 2-CHOME, CHUO-KU, OSAKA-SHI, JAPAN reassignment POLYPLASTICS CO., LTD., 3-13, AZUCHI-MACHI 2-CHOME, CHUO-KU, OSAKA-SHI, JAPAN ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: TSUKADA, MITSUHIRO
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/17—Component parts, details or accessories; Auxiliary operations
    • B29C45/26—Moulds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/17—Component parts, details or accessories; Auxiliary operations
    • B29C45/26—Moulds
    • B29C45/261—Moulds having tubular mould cavities
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
    • B29C45/17—Component parts, details or accessories; Auxiliary operations
    • B29C45/72—Heating or cooling
    • B29C45/73—Heating or cooling of the mould

Definitions

  • This invention relates to a metal mold used to obtain highly accurate injection molded thermoplastic.
  • the invention also relates to tubular or columnar products of molded product obtained using such metal mold.
  • tubular (columnar) tubular (columnar)
  • thermoplastic resin consisting of a thermoplastic resin have been utilized as replacements for metal products since the former can be made relatively easily and efficiently by injection molding techniques.
  • the dimensional accuracy of a tubular molded product to be obtained by conventional injection molding methods is improved generally by regulating the temperature of the metal mold to as uniform a level as possible.
  • attempts to improve the dimensional accuracy of injection molded products have included regulating the temperature of the product core (which easily receives and accumulates therein the heat radiated from the resin, and which constitutes the inner hollow of a final tubular product) with a heat pipe or cooling air.
  • a tubular molded product obtained by this method becomes divergent at the outer circumferences of the end portions thereof as shown in FIG. 5C.
  • the temperature of a metal mold is regulated so as to attain a uniform level and improve the accuracy of a final columnar molded product, it is inevitable that the a product thereby obtained will be deformed due to the shrinkage of an intermediate portion thereof.
  • such a product will be divergent at the circumferential end portions thereof in the same manner as the tubular molded product referred to above.
  • the presently available injection molding technique does not meet in many cases the requirements for the production of a tubular (columnar) part having a high dimensional accuracy, and especially, a high cylindricity.
  • tubular (columnar) products obtained by injection molding are thereafter subjected to further processes, for example, which thereby results in higher manufacturing costs.
  • cutting techniques for producing a tubular (columnar) molded product having high dimensional accuracy are particularly needed.
  • the present invention is directed to a metal mold for injection molding tubular (columnar) products, characterized in that the portions of matching surfaces of metal mold members which are close to end portions of an annular (cylindrical) cavity are provided with grooves extending continuously or intermittently so as to surround the end portions of the annular (cylindrical) cavity.
  • the invention also relates to a tubular (columnar) molded product of a thermoplastic resin molded by this metal mold.
  • FIG. 1 is a schematic section of an embodiment of the metal mold for injection molding tubular (columnar) products according to the present invention
  • FIG. 2 is a schematic section of another embodiment of the present invention, the metal molds shown in FIGS. 1 and 2 being the same metal molds as are used in Examples 1-9 and 10;
  • FIGS. 3A-3D are plan views of examples of grooves provided in the metal mold according to the present invention, with FIGS. 3A-3C specifically showing the groove shapes of the metal mold of FIG. 1 which were evaluated in Examples 1-9, and with the groove shape shown in FIG. 3D being provided in the metal mold of FIG. 2 which was evaluated in Example 10;
  • FIGS. 4A and 4B are schematic diagrams showing the ideas of effective grooves defined in the specification of the present invention.
  • FIGS. 5A and 5B are a perspective view and a sectional view, respectively of a particularly preferred embodiment of this invention.
  • FIG. 5C is a sectional view schematically showing the condition of warp, which causes a decrease in cylindricity, occurring in a tubular molded product obtained by using a conventional metal mold.
  • FIGS. 6-14 show various modes of embodiment of the tubular (columnar) molded products obtained according to the present invention, wherein FIGS. 6A-14A and 6B-14B are perspective views and sectional views, respectively.
  • FIGS. 5A and 5B A preferred tubular molded product is shown in FIGS. 5A and 5B.
  • FIG. 5C schematically shows the shape of an actual molded product which is obtained by molding a resin with a view to forming such a tubular molded product as is shown in FIGS. 5A and 5B, and which is made in a conventional, generally-used metal mold by carrying out the step of cooling the metal mold to a uniform level with the core simultaneously cooled.
  • such a tubular molded product becomes divergent at the outer circumferences of the end portions thereof, so that the outer diameters of the parts of this tubular molded product which are in the vicinity of the end portions 12 thereof become larger than that of an intermediate portion 13 thereof. That is, it is inevitable that the cylindricalness of such a molded product is not sufficiently high level.
  • the injection metal mold used for obtaining a tubular molded product according to the present invention has construction shown in, for example, FIG. 1 or 2.
  • the mold according to the present invention includes an annular cylindrical mold cavity 1 in which the tubular molded products are molded.
  • the mold cavity 1 includes opposing end portions 1a.
  • Molten thermoplastic resin is injected into the cavity 1 via sprue 2 which communicates with the cavity 1 via runner 3.
  • Fluid passages 5 (FIG. 2) and 6 (FIG. 1) are defined in the mold blocks 9 for the purpose of circulating a cooling fluid and thereby regulate the temperature of the mold cavity 1.
  • Parting surfaces 8 are defined between respective faces of the mold blocks 9 and mold blocks 10--that is, the parting surfaces 8 define the transverse end surfaces of the tubular product molded within cavity 1.
  • a heat pipe 7 is provided in operative associated with the mold cavity 1 so as to control the temperature within the mold cavity 1.
  • annular grooves 4 are provided in surrounding relationship to the end portions 1a of the mold cavity 1a. As will be discussed in greater detail below, these grooves serve to equalize the radiant heat loss as between the end portions of the molded product within the cavity 1 and its mid portion so as to prevent shrinkage and/or warpage (resulting in divergent end portions of the molded product). More specifically, the characteristics of the metal mold of the present invention reside in that the portions of the matching surfaces X of the metal mold which are close to the end portions 1a of a cavity 1 for a tubular molded product are provided with grooves 4 surrounding the end portions 1a of the cavity 1 This arrangement reduces the occurrence of the above-mentioned divergent circumferential projections at the end portions of a tubular molded product. Accordingly, a tubular molded product having highly accurate cylindricity is obtained.
  • the matching surfaces X in which the grooves 4 constituting the characteristics of the present invention are formed, are defined by contacting the surfaces between blocks 9 and 10 and core 11 (see FIG. 1).
  • grooves 4 provided in such surfaces is not specially limited. These grooves 4 may be formed to an desire shape, i.e., circular, elliptical, square or polygonal shape. In short, such grooves 4 may be formed to any shape that permits them to be provided so as to continuously or intermittently surround the end portions of the annular mold cavity 1.
  • FIGS. 3A-3D show the examples of the shape of such grooves 4, which are all effective.
  • the shape of the grooves provided in the metal mold according to the present invention is not, of course, limited to those shown in the Figures.
  • a groove 4 provided close to an end portion of the annular cavity 1 performs more effectively. Therefore, it is preferable that grooves 4 be provided so that they are positioned in the portions of the matching surfaces which are within 20 mm, especially, 15 mm from the outer. circumferences of the end portions 1a of the mold cavity 1.
  • the grooves 4 in each of the above-mentioned preferable areas of the matching surfaces be provided so that the sum of the length of each of such arcuate portions portions of the annular cavity in horizontal section that is defined by two straight lines each connecting together the center of this annular cavity and either of the ends of a corresponding groove, is not less than 1/2, especially, 2/3 of a total outer circumference of the annular cavity in horizontal section.
  • grooves of a large width which extend over both the preferable area and the area out of this preferable area, only the portions of the grooves that are within the preferable area may satisfy these conditions.
  • the specially preferable grooves are one or at least two circular grooves formed substantially concentrically with respect to the annular cavity in the portions of the matching surfaces which are within 20 mm, especially, 15 mm from the outer circumference of the annular cavity, for example, such grooves as shown in FIGS. 3A and 3B, which are specially effective.
  • the partially cut grooves shown in FIG. 3C also have substantially the same effect, and are preferable as well.
  • the width of a single groove is preferably not less than 0.15 mm, and more preferably not less than 0.3 mm.
  • the widths thereof are such that the sum of the widths equals one of these levels, and, if the widths of the grooves are set in this manner, the similar preferable effect is obtained
  • the upper limit of the width of the grooves is not specified, and grooves having as large a width as is permitted by the construction of a metal mold can be provided
  • the depth of the grooves in the metal mold according to the present invention be at least 0.1 mm, preferably not less than 0.2 mm, and most preferably not less than 0.3 mm.
  • the upper limit of the depth of the grooves is not more than 1/4 of the length of the annular cavity when the grooves are provided in a molding block (for example, the molding block 10 in FIG. 1) on the side of the annular cavity, and can be set as large as is permitted by the construction of the metal mold when the grooves are provided in a molding block (for example, the molding block 9 and/or 11 in FIG. 1) on the opposite side of the annular cavity.
  • the material for the metal mold according to the present invention is not specially limited, and known material may used. In general, the effect of the present invention can be obtained satisfactorily if the metal mold as a whole is formed out of the same material, for example, structural carbon steel, carbon tool steel, structural rolled steel and stainless steel.
  • a molding block for example, the molding block 10 in FIG. 1 forming the intermediate portion of the tubular (cylindrical) cavity is made of a material the heat conductivity of which is superior to that of the material for the other portion of the molding block. Forming this molding block in this manner also enables a tubular (columnar) molded product of a high dimensional accuracy to be obtained more effectively.
  • substantially the same temperature may be introduced into all the temperature regulating fluid passages 5, 6 as is conventional. Introducing fluids of different temperatures into different fluid passages may also be done, if desired. Especially, introducing a fluid into those passages near the end portions 1a of the cavity 1 having temperature of which is lower than that of the fluid flowing through the fluid passages associated with the intermediate portion of the cavity 2 (for example, the fluid passage 5 in FIG. 5) is preferably in many cases so as to improve the accuracy of the resulting molded product.
  • a conventional core cooling method may also be practiced.
  • a tubular molded product having both highly accurate cylindricity and roundness can be obtained. Accordingly, using such a core cooling method additionally produces a beneficial effect.
  • tubular (columnar) molded products to which the metal mold according to the present invention can be applied will now be described with reference to their illustrations.
  • the tubular (columnar) molded products according to the present invention include not only a product consisting of a complete tubular (columnar) body as shown in FIG. 5 (or FIG. 12) but also a product consisting of combination of a plurality of tubular (columnar) bodies as shown in FIGS. 6 and 7 (or FIG. 13), a product a part of which is cut off, especially, a product a part of which is cut off so that the circumferential width of the cut-off portion is not more than 1/4 of the circumference of the product as shown in FIG. 8 (or FIG.
  • a product consisting of a combination of a tubular body and a columnar body and a product consisting of a tubular body one end of which is closed as shown in FIGS. 9 and 10, respectively, and a product a part of which consists of a tubular (columnar) body integrally formed with the other part thereof as shown in FIG. 11.
  • molded products having a tubular (columnar) part are all included in the molded products according to the present invention.
  • the present invention is effective in improving the dimensional accuracy, especially, the cylindricalness of a tubular (columnar) portion of any of these molded products
  • the present invention has a remarkable effect on a tubular molded product the inner and outer diameters of which are constant throughout the whole thereof as shown in FIG. 5, and a molded product the shape of which is very similar to that of such a molded product.
  • the present invention is more effective on a molded product having a lower wall thickness/outer diameter ratio. Even when this ratio is high, i.e., even in an extreme case where a molded product has a columnar body, the metal mold according to the present invention has a good effect.
  • the molded product in which the degrees of warp and shrinkage are low, and in which the effect of the present invention is specially large, is a tubular molded product the wall thickness/ outer diameter ratio of which is not higher than 1/3.
  • the end portions of the tubular (columnar) body of the resin are hardened earlier and assume dimensions which are closer to those of the corresponding portions of the metal mold.
  • the intermediate portion is hardened later.
  • the resin thus shrinks and assumes a shape having dimensions smaller than those of the corresponding portion of the metal mold.
  • the radiant heat from the end portions of the tubular resin is more limited.
  • solidification rate of the end portions (especially, the circumferences of the end portions) and that of the intermediate portion become almost equal.
  • the resin as a whole shrinks uniformly. It is thus believed that this functional attribute contributes to the highly accurate cylindricity and roundness of the molded product that ensue.
  • the present invention can be applied to any thermoplastic resin so long as it generates molding shrinkage, and is capable of providing highly accurate tubular (columnar) molded products.
  • the present invention has a specially good improvement effect especially on crystalline resins having a high molding shrinkage rate, for example, a polyacetal resin, a polyester resin and a polyamide resin.
  • Tubular products were molded by using metal molds prepared for this purpose and shown in FIG. 1, which molds were provided with grooves 4 of various shapes extending so as to surround the end portions 1a of an annular cavity 1.
  • the details of the shapes of the grooves in the metal molds are as shown in FIGS. 3A-3C, and the sizes thereof as shown in the marginal space of Table 1.
  • the shapes of the grooves were varied in a desired manner by packing solder in predetermined portions of the grooves.
  • the molding blocks were all made of stainless steel, and the tubular molded products obtained were 6 mm in outer diameter, 4.8 mm in inner diameter and 15 mm in length.
  • the resin used in these Examples was a polyacetal resin (Duracon VC-10 manufactured by the Polyplastics Co., Ltd.), and the molding conditions were as follows.
  • tubular products were molded in the same manner by using a metal mold having no grooves on the matching surfaces, and a metal mold having grooves filled completely with solder, and the resultant molded products were evaluated.
  • the results are also shown in Table 1.
  • a groove provided in the portion of a matching surface which is within 1.0-1.5 mm from the outer circumference of an annular cavity, and having a width of 0.5 mm and a depth of 0.5 mm.
  • Intermittent grooves 1.0 mm in width and 0.5 mm in depth provided in the portions of a matching surface which are 1.0-2.0 mm and 3.0-4.00 mm from the outer circumference of an annular cavity respectively.
  • Example 3 A test was conducted under the same conditions as in Example 3 by using a metal mold having the same construction as the metal mold in Example 3 except that the material for the molding block 10, which defines mainly the intermediate portion of the cavity 1 in which a tubular product is molded, was a beryllium-copper alloy having a high heat conductivity.
  • the cylindricalness (difference between the outer diameters of the end and intermediate portions of the annular body) of the molded product obtained was 5 ⁇ m on the gate side and 13 ⁇ m on the far-gate side, which indicated a further improvement in the accuracy of the product.
  • the molding was done in the same manner as in Example 3 except that the temperature of a metal mold, which was the same metal mold as shown in FIG. 1, was regulated by introducing a liquid of 30° C. into a fluid passage 5 and a liquid of 60° C. into a fluid passage 6, and the resultant molded product was evaluated.
  • the cylindricalness (difference between the outer diameters of the end and intermediate portions of the annular body) of this molded product was determined. It was 5 ⁇ m on the gate side and 12 ⁇ m on the far-gate side, which indicated a further improvement in the accuracy of the molded product.
  • a groove 1.00 mm in width and 1.0 mm in depth having the shape shown in FIG. 3D was provided in the portion (gate side alone) designated by a reference numeral 4 and having the shortest distance of 2.00 mm from the outer circumference of the annular cavity, of the matching surface of a metal mold having the same construction as shown in FIG. 2 and designed for obtaining a tubular molded product of 12 mm in outer diameter, 6 mm in inner diameter and 30 mm in length.
  • a polyacetal resin (Duracon M270 manufactured by the Polyplastics Co., Ltd.) was molded under the following molding conditions by using this metal mold, and the molded product obtained was subjected to the measurement of cylindricalness.
  • the size of a core was changed so that the inner diameter of the portion of the metal mold used in Example 10 in which a tubular product was to be molded became 3 mm, and the other conditions were set identical with the corresponding conditions in Example 10.
  • a test was then conducted. The cylindricalness of the molded product obtained was measured. The difference between the outer diameters of the end and intermediate portions of the tubular body was 75 ⁇ m.
  • the cylindricalness of a molded product obtained under the same conditions except that a groove was not provided in the metal mold was 121 ⁇ m.
  • the cylindricalness of a tubular molded product having a smaller inner diameter (larger wall thickness), i.e. being more similar to a columnar molded product is influenced more greatly by the molding shrinkage. Nevertheless, it has been certainly ascertained that providing grooves in a metal mold according to the present invention has excellent effect in minimizing this bad influence.
  • a tubular (columnar) product molded by using a metal mold provided with grooves formed in the portions of the matching surfaces thereof which are close to the end portions of the annular (cylindrical) cavity, in such a manner that the grooves surround the end portions of the cavity, has a very high dimensional accuracy, especially, a very high cylindricalness with the physical properties and outer appearance thereof not spoiled at all.

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  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Mechanical Engineering (AREA)
  • Moulds For Moulding Plastics Or The Like (AREA)
  • Injection Moulding Of Plastics Or The Like (AREA)
US07/333,250 1988-04-15 1989-04-05 Metal mold for injection molding tubular or columnar products and molded product obtained by using same Expired - Fee Related US4973242A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP63092870A JPH0753393B2 (ja) 1988-04-15 1988-04-15 円筒状又は円柱状成形品の射出成形用金型及び成形品
JP63-92870 1988-04-15

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US (1) US4973242A (ja)
EP (1) EP0337805A3 (ja)
JP (1) JPH0753393B2 (ja)
KR (1) KR920002387B1 (ja)
BR (1) BR8901805A (ja)
CA (1) CA1321693C (ja)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5503773A (en) * 1994-09-08 1996-04-02 Genesis Composites, L.C. Method of making a composite handlebar
US20060115551A1 (en) * 2004-11-30 2006-06-01 Hasco-Normalien Hasenclever Gmbh & Co. Injection-mold pin
US20120217722A1 (en) * 2011-01-31 2012-08-30 Juan Carlos Calabresse Muzzi Mold and process for obtaining a bicycle frame made of recyclable polymer
US20190351598A1 (en) * 2018-05-15 2019-11-21 Tyco Electronics (Shanghai) Co. Ltd. Cooling System Of Injection Mold
US11161290B2 (en) * 2015-03-17 2021-11-02 Mitsubishi Heavy Industries, Ltd. Method for manufacturing impeller

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070200279A1 (en) * 2005-01-07 2007-08-30 Toshiya Yui Circular Resin-Molded Product Having Circular Center Hole And Method And Apparatus For Molding The Same
JP2007015291A (ja) * 2005-07-08 2007-01-25 Olympus Imaging Corp 筒状部品
JP4912091B2 (ja) * 2006-08-31 2012-04-04 株式会社リコー 光ディスク基板成形用金型
JP4600461B2 (ja) * 2007-11-06 2010-12-15 三菱電機株式会社 成形品の製造方法
JP2011016378A (ja) * 2010-10-13 2011-01-27 Olympus Imaging Corp 筒状部品
JP2016150461A (ja) * 2015-02-16 2016-08-22 小島プレス工業株式会社 金型装置

Citations (5)

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US4101699A (en) * 1973-11-23 1978-07-18 Samuel Moore & Company Synthetic resinous tube
US4126291A (en) * 1974-10-18 1978-11-21 California Injection Molding Co., Inc. Injection mold for elongated, hollow articles
US4483891A (en) * 1981-12-11 1984-11-20 Ball Corporation Multilayered tube having a barrier layer
US4707389A (en) * 1983-09-30 1987-11-17 Baxter Travenol Laboratories, Inc. Multilayer tube, assembly and method
US4787596A (en) * 1987-03-06 1988-11-29 Hewlett-Packard Company Mold for forming plastic Luer nuts

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JPH07102625B2 (ja) * 1982-04-13 1995-11-08 松下電器産業株式会社 円盤状記録媒体の成形装置
JPS5997256A (ja) * 1982-11-26 1984-06-05 Fujitsu Ltd 回線接続装置の接続方式
JPS60182124U (ja) * 1984-05-15 1985-12-03 不二精機株式会社 テ−プリ−ル金型
JPH0745154B2 (ja) * 1986-10-31 1995-05-17 オリンパス光学工業株式会社 プラスチツクレンズ成形用金型
JPH0712635B2 (ja) * 1987-09-28 1995-02-15 ポリプラスチックス株式会社 円筒状成形品の射出成形方法

Patent Citations (5)

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Publication number Priority date Publication date Assignee Title
US4101699A (en) * 1973-11-23 1978-07-18 Samuel Moore & Company Synthetic resinous tube
US4126291A (en) * 1974-10-18 1978-11-21 California Injection Molding Co., Inc. Injection mold for elongated, hollow articles
US4483891A (en) * 1981-12-11 1984-11-20 Ball Corporation Multilayered tube having a barrier layer
US4707389A (en) * 1983-09-30 1987-11-17 Baxter Travenol Laboratories, Inc. Multilayer tube, assembly and method
US4787596A (en) * 1987-03-06 1988-11-29 Hewlett-Packard Company Mold for forming plastic Luer nuts

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5503773A (en) * 1994-09-08 1996-04-02 Genesis Composites, L.C. Method of making a composite handlebar
US20060115551A1 (en) * 2004-11-30 2006-06-01 Hasco-Normalien Hasenclever Gmbh & Co. Injection-mold pin
US20120217722A1 (en) * 2011-01-31 2012-08-30 Juan Carlos Calabresse Muzzi Mold and process for obtaining a bicycle frame made of recyclable polymer
US11161290B2 (en) * 2015-03-17 2021-11-02 Mitsubishi Heavy Industries, Ltd. Method for manufacturing impeller
US20190351598A1 (en) * 2018-05-15 2019-11-21 Tyco Electronics (Shanghai) Co. Ltd. Cooling System Of Injection Mold

Also Published As

Publication number Publication date
JPH0753393B2 (ja) 1995-06-07
EP0337805A2 (en) 1989-10-18
KR920002387B1 (ko) 1992-03-23
KR900015877A (ko) 1990-11-10
BR8901805A (pt) 1989-11-28
JPH01263016A (ja) 1989-10-19
CA1321693C (en) 1993-08-31
EP0337805A3 (en) 1991-06-12

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